Optical information recording/reproduction apparatus (optical disk drive) for recording/reproducing information on/from an optical information recording medium (optical disk) have been popular in use. An optical disk drive encodes the data to be recorded by means of any of a variety of coding systems and classifies the data into a plurality of marks and a plurality of spaces to have a binary state before actually recording the information on the optical disk. When reproducing data, it detects the mark length/space length of each of the marks/spaces formed on the optical disk from the reproduced signal to read the recorded data and decodes the data by means of the appropriate coding system before actually reproducing the data. It is to be noted that the symbol “/” such as used in “recording/reproducing apparatus” and “mark length/space length” means “and/or” in this text.
When recording data on an optical disk, the mark length/space length to be formed can often be different from the actually formed mark length/space length due to thermal interference etc. If the mark length/space length formed on an optical disk differs from the mark length/space length to be formed, a jitter phenomenon takes place at the time of data reproduction and adversely affects the data reproduction performance. When the jitter is significant, an error state occurs frequently to reduce the reproduction quality of the reproduced data. Such a situation is described in detail in the Republished Patent WO2002/084653. According to the Patent Publication, the edge shift quantity of a mark formed on an optical disk is determined by measuring the jitter of a reproduced signal and the recording parameters are adjusted according to the determined edge shift quantity.
The above Patent Publication shows an example of using PRML (partial response most likelihood), according to which the equalization error of a PR-equalized signal from level-0 is detected at each crossing (0-cross) of a central reference level (level-0) and hence at each polarity inversion of a PR-equalized signal. At this stage, with respect to combinations of mark and space, the marks and space are classified into mT marks (m is an integer not less than 1, and T is a channel clock) and nT spaces (n=an integer not less than 1, T=a channel clock) to detect an equalization error. Subsequently, the jitter quantity is determined by determining the cumulative value of the equalization errors from level-0, each of which takes place at each polarity inversion, and dividing the number of occurrences of equalization errors by the cumulative value. Thereafter, the position of the leading edge of the mark to be recorded and the position of the trailing edge of the mark to be recorded are adjusted to determine the edge shift quantity that minimizes the jitter at the time of reproduction and optimizes the recording conditions. Then, the mark is recorded again under the optimized recording conditions. The inventor of the present invention investigated degradation of the reproduction quality of reproduced data in the manner as described hereinafter.
According to the above Patent Publication, the cumulative value of the equalization errors, each of which is observed when a PR-equalized signal crosses level-0, is determined to by turn determine the jitter. The operation of determining the cumulative value of the equalization errors from level-0, each of which takes place as the level difference of an PR-equalized signal from level-0, is meaningful when the clock sampling point of an ideal waveform agrees with a central reference level (level-0) in a transition from a mark to a space or vice versa for a PR class. However, the clock sampling point of an ideal waveform does not necessarily agree with a central reference level (level-0) depending on the PR class. If such is the case, it is not possible to correctly measure the jitter by means of the technique described in the above Patent Publication.
FIG. 13 schematically illustrates how a 3 T repetitive waveform shifts for PR1221. FIG. 14 schematically illustrates how a 3 T repetitive waveform shifts for PR121 or PR12221. As shown in FIG. 13, the ideal waveform is at a level-0 at the time of sampling thereof for PR1221. However, as shown in FIG. 14, the ideal waveform is not at level-0 at the time of sampling thereof for PR121 and PR12221. According to the above Patent Publication, it is not possible to correctly determine the jitter and the recording conditions cannot be appropriately adjusted for a PR class where the ideal waveform is not always at level-0 at the time of sampling thereof as shown in FIG. 14.
Thus, the technique described in the above Patent Publication is applicable to a PR class where the ideal waveform is at level-0, whereas the technique is not applicable to any PR class where the ideal waveform is not at level-0. If the difference between the position and the length of the formed mark/space and the position and the length of ideal mark/space can be determined from a reproduced signal, the recording conditions can be adjusted equally in a similar way for PR classes where the ideal waveform is at level-0 and for PR classes where the ideal waveform is not at level-0.